Journal of Non-Newtonian Fluid Mechanics

Complex fluid flow in porous media is ubiquitous in many natural and industrial processes. Direct visualization of the fluid structure and flow dynamics is critical for understanding and eventually manipulating these processes. However, the opacity of realistic porous media makes such visualization very challenging. Micromodels, microfluidic model porous media systems, have been developed to address this challenge. They provide a transparent interconnected porous network that enables the optical visualization of the complex fluid flow occurring inside at the pore scale. In this Review, the materials and fabrication methods to make micromodels, the main research activities that are conducted with micromodels and their applications in petroleum, geologic, and environmental engineering, as well as in the food and wood industries, are discussed. The potential applications of micromodels in other areas are also discussed and the key issues that should be addressed in the near future are proposed.

Microfluidic Model Porous Media: Fabrication and Applications.

A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number Wi(crit) ≈ 0.5. Birefringence and bulk pressure drop measurements provide self-consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates (26 s(-1) ≤ ε ≤ 435 s(-1)) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model.

Optimized Cross-Slot Flow Geometry for Microfluidic Extensional Rheometry

Fluid vesicles in flow.

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Active structuring of colloidal armour on liquid drops

Drop deformation in an uniform dc electric field is a classic problem. The pioneering work of Taylor demonstrated that for weakly conducting media, the drop fluid undergoes a toroidal flow and the drop adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. However, recent studies have revealed a nonaxisymmetric rotational flow in strong fields, similar to the rotation of solid dielectric particles observed by Quincke in the 19th century. We present a systematic experimental study of this phenomenon, which highlights the importance of charge convection along the drop surface. The critical electric field, drop inclination angle, and rate of rotation are measured. We find that for small, high viscosity drops, the threshold field strength is well approximated by the Quincke rotation criterion. Reducing the viscosity ratio shifts the onset for rotation to stronger fields. The drop inclination angle increases with field strength. The rotation rate is approximately given by the inverse Maxwell–Wagner polarization time. Novel features are also observed such as a hysteresis in the tilt angle for large low-viscosity drops.

Electrohydrodynamics of drops in strong uniform dc electric fields

Viscoelastic flow past a cylinder is a classic benchmark problem that is not completely understood. Using novel three-dimensional (3D) holographic particle velocimetry, we report three main discoveries of the elastic instability upstream of a single cylinder in viscoelastic channel flow. First, we observe that upstream vortices initiate at the corner between the cylinder and the wall, and grow with increasing flow rate. Second, beyond a critical Weissenberg number, the flow upstream becomes unsteady and switches between two bistable configurations, leading to symmetry breaking in the cylinder axis direction that is highly 3D in nature. Lastly, we find that the disturbance of the elastic instability propagates relatively far upstream via an elastic wave, and is weakly correlated with that in the cylinder wake. The wave speed and the extent of the instability increase with Weissenberg number, indicating an absolute instability in viscoelastic fluids.

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Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder

We report a novel non-invasive method to measure the capacitance of biomimetic bilayer membranes. The approach utilizes the frequency-dependent deformation of giant unilamellar vesicles in AC uniform electric field. The method is applied to membranes made of lipids and polymers. Compared to lipid membranes, polymer bilayers have an order of magnitude lower capacitance, which correlates with their larger thickness. The capacitance of both lipid and polymer bilayers is found to be voltage-independent in the examined range between 2 and 6 kV m−1, indicating membranes do not experience significant thinning due to electrostriction.

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Electrodeformation method for measuring the capacitance of bilayer membranes

The transient deformation of giant vesicles in square DC electric pulses is investigated. We experimentally observe the theoretically predicted transition from an oblate to prolate ellipsoidal shape in the case of a quasi-spherical vesicle encapsulating solution less conducting than the suspending medium. The transition is detected by utilizing a two-step pulse in order to avoid electroporation and vesicle collapse. We develop a theoretical model to describe both the deformation under an electric field and relaxation after the field is turned off. Agreement between experiment and theory demonstrates that the time-dependent vesicle shape can be used to measure membrane properties such as viscosity and capacitance.

Vesicle deformation in DC electric pulses.

We report a fluid system which exhibits chaotic dynamics under creeping flow conditions. A droplet in a uniform dc electric field deforms into an ellipsoid that can undergo irregular rotational motions. The nonlinear drop electrohydrodynamics is explained by a theoretical model which includes anisotropy in the polarization relaxation due to drop asphericity and charge convection due to rotational drop flow.

Paul Salipante

Papers

Electrohydrodynamics of drops in strong uniform dc electric fields

Upstream vortex and elastic wave in the viscoelastic flow around a confined cylinder

Electrodeformation method for measuring the capacitance of bilayer membranes

Vesicle deformation in DC electric pulses.

Electrohydrodynamic rotations of a viscous droplet.